1. Field of the Invention
The present invention relates to an optical modulator used in an optical communication system, and particularly to a Mach-Zehnder optical modulator.
2. Description of the Related Art
In the field of optical communication systems that are capable of transmitting a large volume of information, further improvements in transmission rates are considered crucial, and high-speed modulators are considered to be key devices for improving transmission rates.
One optical modulator that is capable of high-speed modulation has an optical waveguide structure that uses a Mach-Zehnder interference system. Such a Mach-Zehnder optical modulator is frequently used particularly as an external modulator in ultra-high-speed communication systems not only because it is capable of canceling the in-phase component in the noise component by applying drive voltage in a push-pull mode, but is also stable with respect to disturbance and can obtain modulation characteristics featuring excellent S/N (signal-to-noise ratio).
In a Mach-Zehnder optical modulator, input light is split into two beams which each undergoes phase modulation and then are combined. In this way, modulation of light intensity is effected by mutual interference. A Mach-Zehnder optical modulator is normally constructed such that the phase difference between the two optical waveguides that propagate the split beams is 0 when voltage is not being applied. In this type of optical modulator, therefore, input light is outputted without change when the applied voltage is 0, and the intensity of the output light varies as a cosine curve as voltage is applied. When operating this type of optical modulator, however it is desirable for the radiated light intensity to vary linearly with respect to the intensity of the electric field that is applied from the outside. For this reason, the initial operating point in a Mach-Zehnder optical modulator is typically set to a position where the phase is shifted by xcfx80/2 radian (90 degrees).
Referring now to FIG. 1A, in which is shown the construction of a typical Mach-Zehnder optical modulator, imbedded optical waveguide 82 is provided in optical substrate 81, which has an electro-optical effect. In optical waveguide 82, input waveguide 82a is branched into two optical waveguides 82b and 82c by way of the Y-shaped branching portion, following which branch optical waveguides 82b and 82c are joined by way of the Y-shaped combining portion, thereby constituting a Mach-Zehnder interference system waveguide. Optical buffer layer 89 and a travelling wave electrode 84 of a prescribed pattern are further provided on branch optical waveguides 82b and 82c. 
In this optical modulator, linearly single polarized light that is applied to input waveguide 82a is equally divided at the Y-shaped branching portion and advances into optical waveguides 82b and 82c. At this time, electric fields that are generated in optical waveguides 82b and 82c by applying voltage to travelling wave electrodes 84 as shown in FIG. 1B are applied to optical waveguides 82b and 82c in mutually opposite vertical directions. As a result, the refractive indexes of each of optical waveguides 82b and 82c change due to the electro-optical effect of optical substrate 81, the change in refractive index in each of optical waveguides 82b and 82c being equal in amount and acting oppositely according to the positive and negative sign. The phase modulation due to the change in refractive index thus works in a push-pull manner on optical waveguides 82b and 82c. 
The light waves that receive the phase modulation (xc2x1xcfx86/2) of these optical waveguides 82b and 82c are combined by the Y-shaped combining portion, mutually interfere, and then proceed to output waveguide 82d to be outputted from the output terminal. In this case, the intensity of the output light is altered by cos2 (xcfx86/2) with respect to the total amount of phase modulation xcfx86. For example, when light waves that are guided by optical waveguides 82b and 82c are subject to combination/interference, output is at a maximum when the light waves are of the same phase (xcfx86=2n xcfx80) and at a minimum when of the opposite phase (xcfx86=(2n+1)xcfx80). In this case, n is an integer such as 0, 1, 2 . . .
Typically, when carrying out light intensity modulation in the Mach-Zehnder interference optical modulator shown in FIG. 1A, the initial operating point is preferably set to an intermediate position (xcfx80/2 phase state) between the maximum output and the minimum output. A device has therefore been proposed in which dc power supply 85 and bias circuit 86 are provided in addition to high-frequency power supply 87 as shown in FIG. 2A in order to allow regulation of the initial operating point. According to this configuration, a direct current (dc) voltage for setting bias is applied to travelling wave electrode 84 in addition to the modulation signal voltage (ac voltage), which is the driving voltage, whereby the refractive index of the optical waveguide changes due to the electro-optical effect of the optical substrate and the phase shifts. FIG. 2B shows the output characteristic in this optical modulator when the dc voltage is 0.
As another method in which the initial operating point of an optical modulator is adjusted to xcfx80/2 phase state, Japanese Patent Laid-open No. 297332/93 (JP, 05297332, A) discloses a device in which the two branch optical waveguides that constitute a Mach-Zehnder interference system are formed with slightly differing lengths, thus producing a difference in the lengths of the optical paths between these two optical waveguides that is on the optical wavelength order.
As yet another known method of adjusting the initial operating point of an optical modulator, the width of the two optical waveguides that constitute a Mach-Zehnder interference system may be varied in portions to produce a non-symmetrical shape, thereby bringing about a difference in the effective refractive index and shifting the initial operating point (Tsuchiya, Kubota, Seino; Papers of the 1992 Autumn Conference, Institute of Electronics, Information and Communication Engineers, C-171). In addition, Japanese Patent No. 2,564,999 (JP, 2564999, B1)(corresponding to Japanese Patent Laid-open No. 24327/92 (JP, 04024327, A)) discloses a method in which the two optical waveguides that constitute a Mach-Zehnder interference system are coupled by a 3-dB directional coupler to shift the optical output by xcfx80/2 radian.
Nevertheless, the above-described optical modulators of the prior art have the following problems:
In an optical modulator in which the adjustment of the initial operating point is performed by applying dc voltage in addition to the modulation signal voltage (ac voltage), a stable modulation characteristic cannot be maintained over a long time period due to change of the operating point with the passage of time, i.e., the dc drift phenomenon. This dc drift phenomenon is often observed when, for example, a lithium nobate (LiNbO3) crystal are used in the optical substrate. Moreover, the bias circuit that is necessary for applying the dc voltage takes up space and adds to the expense of the device. Furthermore, the initial operating point and phase of the output light of the optical modulator can be kept uniform by providing a feedback circuit for correcting the voltage to follow up change in the electric field due to the dc drift phenomenon, thereby stabilizing the modulation characteristic, but taking such measures not only increases cost but adds to the complexity of the circuit configuration.
Optical modulators in which physical differences in the optical paths are created by varying the length or shape of the two optical waveguides that constitute the Mach-Zehnder interference system allow open control for the drive circuit of each optical modulator and thus have real advantages because they have a simplified configuration and have few components in their electrical circuits, but since these optical modulators are constructions for creating phase differences, they also cause differences in optical loss between the two optical waveguides and moreover, the propagation speed of light waves lacks spatial uniformity. Such devices therefore suffer from problems such as: deterioration in extinction ratio, i.e., the ratio of maximum value to minimum value of emitted light intensity; decrease in the emitted light intensity; and further, the occurrence of divergence in speed matching between microwaves and light waves.
A device in which the two optical waveguides that make up the Mach-Zehnder interference system are joined by a 3-dB directional coupler can be expected to provide a solution for each of the above-described problems, but it has been found based on the results of experimentation thus far that such a device suffers from the problems that the extinction ratio deteriorates and the initial operating point diverges from a xcfx80/2 phase state when a difference occurs between the propagation constants of the two optical waveguides.
In addition to the problems described in the foregoing explanation, each of the optical modulators of the prior art suffers from the following problems relating to temperature drift:
The material (for example, lithium niobate) that makes up the optical substrate that is used in this type of optical modulator has pyroelectric properties, and if the temperature of the optical substrate rises due to changes in the ambient temperature, the pyroelectric effect produces a polarization charge in the substrate surface. This polarization charging disrupts the uniformity of the electric field that is applied to the two branch optical waveguides that make up the Mach-Zehnder interference system, thereby causing a change in the refractive index of the two optical waveguides. As a result, a stable modulation characteristic cannot be maintained.
The optical substrate also possesses piezoelectric properties. An increase in the temperature of the optical substrate gives rise to stress in the substrate, this stress resulting from the temperature dependence of internal stress that is caused by forming an optical buffer layer or surface electrodes. As a result, a polarization charge is also produced on the substrate surface by the piezoelectric effect. As with the above-described pyroelectric effect, a change is produced in the refractive index of the two branching optical waveguides, and stable modulation characteristics cannot be maintained.
Furthermore, the above-described stress that is produced in the optical substrate causes a difference in the optical paths of the two branch optical waveguides, whereby stable modulation characteristics cannot be maintained. This problem of stress is also produced by the difference in the thermal expansion rates of the case and optical substrate.
It is therefore an object of the present invention to provide an optical modulator that solves the above-described problems, that can realize a suitable initial operating point without using dc voltage for control, and moreover, that has little optical loss and can obtain an excellent extinction ratio.
It is another object of the present invention to provide an optical modulator that can prevent temperature drift.
In an optical modulator that uses optical waveguides based on a Mach-Zehnder interference system, a configuration is adopted in which the two branch optical waveguides that make up the interference system are joined by a 3-dB directional coupler, whereby control can be effected to shift the operating point by xcfx80/2 phase in advance, and ideally, the need to apply a bias voltage can be completely eliminated. A bias-free configuration can thus be realized.
In an optical modulator in which the first and second branch optical waveguides that make up the Mach-Zehnder interference system are joined by a 3-dB directional coupler, the present inventors conducted research and found that the extinction ratio is dependent on the ratio of nonuniformity, which is represented by the difference between the propagation constants of the two optical waveguides that make up the 3-dB directional coupler, to the coupling coefficient of the 3-dB directional coupler. When the practical extinction ratio in the present invention is assumed to be 20 dB, to achieve an extinction ratio of this level, the ratio of the nonuniformity to the coupling coefficient is 1 to at least 5, preferably 1 to at least 15, and still more preferably 1 to at least 20. In contrast to the prior art, no shift occurs in the phase of the output light and initial operating point of the optical modulator of the present invention that is configured as described above, and the modulation characteristics therefore do not become unstable.
The present inventors also found that a bias-free construction can be realized by using a multimode interference (MMI) optical waveguide in place of the 3-dB directional coupler in an optical modulator that uses Mach-Zehnder interference-type optical waveguides. A construction that employs a MMI optical waveguide is free of a problem that is characteristic of a 3-dB directional coupler, this problem being shifting of the initial operating point and phase of the output light of the optical modulator and the resulting instability of modulation characteristics.
For optical modulation in the present invention, it is preferable that a first ground electrode is provided over the first branch optical waveguide, a signal electrode is provided over the second branch optical waveguide, and a second ground electrode is provided at a position symmetrical to that of the first ground electrode such that the signal electrode is interposed between two ground electrodes. By means of this construction, a substantially uniform electric field is generated from the signal electrode to each of the first and second ground electrodes, whereby the electric field that is applied to the first and second branch optical waveguides that constitute the Mach-Zehnder interference system can be made uniform. Adopting a symmetrical configuration of the electrodes also equalizes the effect on each branch optical waveguide that is caused by the stress on the optical substrate that is produced by the load of these electrodes.
In addition, a portion of the second ground electrode can be formed over the optical waveguides that make up the 3-dB directional coupler or over the multimode interference optical waveguide. By adopting this structure, electrodes that are provided to cover the optical waveguide can block the influence of disturbances such as temperature change or charging.
The mutually opposite phase relation or the equal power relation of the two optical outputs that are outputted from the 3-dB directional coupler or the 2-input 2-output multimode interference optical waveguide can be used when modulating light using the optical modulator according to the present invention. In other words, a desired optical output of the two optical outputs can be extracted. The user can select one optical output of the two optical outputs according to necessity, or, if necessary, the user can extract and use both of the optical outputs. This optical modulation method therefore offers a high degree of freedom in design.
The present invention can offer the advantages of lower cost and smaller size because it does not require circuits, such as a power supply for bias dc voltage, a bias circuit, or a feedback circuit, that were necessary in each modulator of the prior art. In addition, the invention allows a reduction in the influence of dc drift and temperature drift and therefore can offer not only stabilized modulation characteristics over a long period of time but has a low level of optical loss and can obtain an excellent extinction ratio. As a result, the present invention can provide an optical modulator with superior reliability that was not available in the prior art. The provision of an optical modulator having both high performance and high reliability can be considered an extremely important merit of the present invention.
The above and other objects, features, and advantages of the present invention will become apparent from the following description based on the accompanying drawings which illustrate examples of preferred embodiments of the present invention.